Method and apparatus for floating-zone melting of semiconductor material



Aug. 9, 1966 w. KELLER METHOD AND APPARATUS FOR FLOATING-ZONE med Atg. 12, 1960 MELTING OF SEMICONDUCTOR MATERIAL 3 Sheets-Sheet 1 wan Fig.1

W. KELLER PAR Aug. 9, 1 966 3,265,470 METHOD AND AP ATUS FOR FLOATING-ZONE MELTING OF SEMICONDUCTOR MATERIAL 3 Sheets-Sheet 2 Filed Aug. 12,- 1960 Fig. 4

Aug. 9, 1966' w. KELLER METHOD AND APPARATUS FOR FLOATINGZONE MELTING OF SEMICONDUCTOR MATERIAL 3 Sheets-Sheet 3 Filed Aug. 12. 1960 7 m 1 w 1 1 a a -1 n M w n ll 1 0 B n Tm m u ,1 6 2 1 m a. F w 9 O 0 1 Fig. 7

United States Patent 'METHQD AND APPARATUS FOR FLOATING-ZONE MELTING OF SEMICGNDUCTOR MA'IERIAL Wolfgang Keller, Pretzfeld, Upper Francouia, Germany,

assignorto. Siemens-Schuckertwerke Aktiengesellschaft, Berliu-Siemensstadt, Germany, a corporation of Ger- My invention relates to a method and apparatus for the crucible-free, floating-zone meltin of semiconductor material aceor'dingto which a melting zone between the two ends of a semiconductor rod is passed longitudinally along the rod, the zone being heated by means of an inductance coil which surrounds. the rod and which is energized by a high-frequency generator.

My invention relates more particularly to a method and apparatus for processing semiconductor rods disclosed in the application, Serial No. 806,174, filed April 13, 1959,- of Th. Rummel, Keller and H. F. Quast, now Patent No. 2,913,561, granted November 17, 1959. According to this method the diameter-dependent heating current to the inductancecoil is used for controlling a device capable of varying the spacing between the two holders in which the respective rod ends are held. Said controlled device moves the two holders toward or away from eachother until the current flowing in the highfrequency coil, upon departure from a datum value, again assumes that value.

It is an object of my invention to further improve the above-mentioned method to obtain better facility and reliability of automatic control and regulation to produce a rod of predetermined and constant diameter.

According to a feature of my invention, the desired rod diameter is secured by controlling and varying the frequency of the high-frequency generator furnishing the current for energizing the induction heater coil that produces the travelling molten zone.

According to another feature of my invention, the

desired rod diameter is obtained by controlling and varying the heating circuit frequency or heating current power while changing the capacitance of the heating circuit. According to still another feature of my invention, the desired rod diameter is obtained by any type of variation in the relation between the oscillatory generator circuit and the heating circuit. variation of the coupling between the generator circuit and heating circuit, While maintaining the other parameter values of the two circuits constant.

The invention will be further described with reference to the accompanying drawings in which:

FIG. 1 illustrates, schematically, a device and circuit diagram of equipment for controlling and regulating the rod diameter, according to the invention. 7

FIG. 2 is the circuit diagram of a high-frequency generator applicable in a control and regulating system ac cording to FIG. 1.

FIG. 3 is an explanatorydiagram relating to resonance conditions in the induction heating circuit.

FIG. 4 illustrates a modified detail in a device otherwise as shown in FIG. 1.

FIG. 5 is a graph explanatory of a control program applicable for the purposes of the invention.

f FIG. 6. illustrates schematically another apparatus for performing the method according to the invention.

FIG. 7 and FIG. 8 illustrate modifications of the in duction heating circuit applicable in apparatus according to FIG. 1 or FIG. 6.

I include in such types the 1 by induction heating.

3,265,470 Patented August 9, 1965 "ice According to FIG. 1, a semiconductor rod 2, such as silicon, has its ends firmly clamped in respective holders 3, 4. The upper holder 4 is axially displaceable. The lower holder 3 is axially fixed but may be rotatable about its axis. An induction heater coil 5 surrounds an axially narrow zone 6 of the semiconductor rod for melting it For compensating the reactive current, a capacitor 7 is connected parallel to coil 5. The coil 5 and the capacitor 7 form together a secondary oscillatory (tank) circuit, hereafter called heating circuit which is coupled to the output terminals 8 of a high-frequency generator 9. The high-frequency generator preferably operates on a flank of the resonance curve of the heating circuit.- The generator 9 is energized at its input terminals 10 from a direct-current supply. A rotatable knob 11 permits adjusting the frequency of the generator. A high-frequency generator suitable for the purposes of theinvention will bedescribed below with reference to FIG. 2 and its performance will be explained with reference to FIG. 3.

The anode (output) current of the high-frequency generator 9 is dependent upon the power consumed by the heating circuit and hence is also dependent upon the diameter of the semiconductor. rod 2, because a change in rod diameter causes a change in the degree of coupling between the inductance coil 5 and the molten zone 6. Such a variation of high-frequency current isindicated by an instrument 12 and produces a corresponding voltage drop along a resistor 13. This resistor is to be adjusted so that the generator output current obtaining at the desired diameter of the semiconductor rod, produces in resistor 13 a voltage drop equal to the voltage of a compensating' voltage source or battery 14. As long as such compensation is effected, a polarized relay 15 in the cornpensating' circuit is inactive so that its cont-acts 16 are open.

When the rod diameter changes from the desired value, the anode or output current of the generator 9 also changes and hence causes a corresponding change in voltage drop of resistor 13. As a result, the relay 15 attracts its armature (not shown) and the contacts 16 connect a reversible motor 17 for rotation in a given direction. The motor 17 drives a pinion 18 meshing with a rack 19, which displaces the upper rod holder 4, so that the upper holder 4 moves either downward or upward. By suitable polarization of relay 15 and motor 17, the motor 17 will always run in the proper direction, namely so that when the rod diameter tends to become too large, a stretching of the melting zone occurs, whereas the melting zone is narrowed and widened when the rod diameter tends to become too small. In this manner, a defined and predetermined constant diameter of the semiconductor rod is secured during zone melting operation.

Thorough investigation of the above described method has resulted in the finding that for a given heating circuit, and a constant coupling between heating circuit and high-frequency generator, there exists a definite relation between generator frequency, anode (output) current of the generator and rod diameter. Consequently, by adjusting given frequency values of generator output cur rent, correspondingly definite, reproducible rod diameters can be reliably secured. It has further been found that the rod diameter reacts much more intensively upon a changing frequency than upon a changing current. Thus a frequency change of 1% upwardly or downwardly resulted in a change of diameter by about 6%. In contrast, a change in anode current by 5% results in a change of the rod diameter by only about 1%. Based upon this discovery, it is a preferred feature of my invention to utilize frequency control of the highfrequency generator for controlling and regulating the desired rod diameter. The frequency scale of the generator, as shown near knob 11in FIG. 1, can then be calibrated directly in terms .of rod diameter.

FIG. 2 illustrates details of a high-frequency generator suitable for the invention. The high-frequency generator is energized at terminals 10 from a direct-current source of constant voltage and includes a tank circuit consisting of a capacitor 20 and an inductance coil 21. This tank circuit determines the frequency of the generator output voltage, and is connectedin the plate circuit of a triode 22. The above mentioned lR dr op resistor 13 is connectedin the plate circuit in series relation to the cathode of tube 22. As mentioned, the voltage-drop of resistor 13 is proportional to the anode current of the generator. The heating circuit, comprising the heater coil and the capacitor 7, is coupled with the generator circuit, for example inductively, as shown in FIG. 2. For this purpose, the inductance coil 21 is provided with a secondary winding 210 from which the heating circuit is energized. The capacitor 20 is adjustable and may be designed as a rotary capacitor device whose rotary electrode member is connected with the knob'll shown in FIG. 1. A high-frequency range of 1 to 5 megacycles per' second is particularly suitable for the zone meltingoperation. The diameter of the rod to be processed may be 18 mm., for example.

As mentioned above, for best performance the highfrequency generator is preferably operated on a flank of its resonance curve. The resonance curve is exemplified in FIG. 3 where the abscissa denotes frequency (f), and the ordinate denotes current I or voltage U. The resonance curve may represent the frequency characteristic of the heating circuit 5, 7. The high-frequency generator preferably operates not in the direct vicinity of the resonance frequency fr, but along a flank portion of the resonance wave, preferably the frontal flank as shown in FIG. 3. When the generator frequency fg increases, the current in the heating circuit likewise increases. When the generator frequency ,fg declines, the heating current and hence the heating power likewise declines. If the high-frequency generator did not operate on a flank of the resonance curve, but in the vicinity of the resonance point fr, of the heating circuit, the just-mentioned substantially linear dependence of the heating current upon the frequency would not be prescut, because a reversal of this dependence would occur whenever the operating point passes beyond the resonance point. I I According to the prior discolsure embodied in the copending application Serial No. 794,075, filed February 18, 1959, of Wolfgang Keller for Method for Producing Semiconductors assigned to the assignee of the present invention, a m-onocrystal can be pulled from a polycrystalline semiconductor rod by floating zone melting when fusing to one end of the polycrystalline rod a monocrystalline seed whose cross section is considerably smaller than that of the rod to be processed. This method requires passing a melting zone repeatedly from the seed along the r od. This method, being an improvement of a similar method in which the seed crystal has the same or nearly the same diameter as the semiconductor rod to be processed, has the advantage that the transfer of heat from the melting zone through the small cross section of the seed to the rod ,holder is greatly reduced with the result that the temperature gradient, in the rod portion immediately adjacent to the fusion junction between seed and rod is diminished. This in turn reduces the occurrence of lattice defections in the recrystallizing semiconductor material, thus improving the qu-ality of the product.

.However, if the method according to the present invention is performed while thus using a seed crystal of much smaller cross section than the semiconductor rod, difiiculties are encountered. The shape of the transition between the semiconductor rod of normal thickness and the said much thinner seed crystal is not preserved. According to the above described. regulation for obtaining constant rod cross section, the seed crystal becomes thickened. After several passes of the melting zone, the cross section of the seed crystal is equalized with the cross section of the rod proper, thus sacrificing the above-mentioned advantages of the thin seed crystal,- namely the reduced heat losses, the reduced transfer of impurities from the seed crystal to the rod, and the reduced issuance of lattice defections from the v.seed crystal into the rod.

One way of avoidingthese difiiculties is the following. At the beginning of the zonemelting process, the device which displaces the two rod holders toward or away from each other is made inactive and the generator frequency and/or the current supplied to the inductance heater coil is adjusted by hand while observing the melting zone. Thereafter the method according to the present invention for obtaining a constant and predetermined rod cross section isv initiated only after the melting zone has travelled out of the range where the seed crystal is fused to the rod. This is to be repeated at the beginning of each following pass of the melting zone.-

It will be realized that although such operation is feasible, the use of a seed crystal having a considerably smaller cross section than the semiconductor rods would tend to cause appreciable trouble when employingan automatically operated control and regulating method according to the invention. It is therefore another object of my invention to eliminate such difficulties.

To this end, and in accordance with another feature of my invention, the transition of the melting zone from the thin seed crystal to the thicker semiconductor rod is controlled by an auxiliary device in accordance with a predetermined control program governing the relation of the. generator frequency to the current supplied to the inductance heater coil.

This will be further explained with reference to FIGS. 4, 5 and 6.

FIG. 4 shows the semiconductor rod 102, the seed crystal 103 of much smaller diameter cross section, and the induction heater coil 104 which surrounds a melting zone 105. The heater coil is preferably a spiral coil, comprising one coil layer in the axial direction, as described in the Keller and Emeis co-assi-gned application Serial No. 23,535 filed April 20, 1960. It may consist of copper tubing traversed by cooling water during operation. An arrow x indicates the travelling direction of the heater coil 104 relative to the semiconductor rod 102 during a zone-melting pass.

The diagram according to FIG. 5 indicates the required relation between the generator frequency fg, or the heating current I to the travel path x of the heater coil, within a travelling range extending from the fusion junction of the rod with the thin seed crystal (x up to the location (x in the rod where, for obtaining stable conditions of the melting zone, the full constant cross section of the semi-conductor rod 102 is reached.

At first, a melting zone is produced in the vicinity of the thin seed crystal 103. For this purpose, the heater coil is supplied with current corresponding to the initial current value shown at location x of the diagram. The generator frequency jg generally also coresponds to the value shown at the same location x in FIG. 5. For maintaining the heating current I constant, the generator frequency is regulated and hence fluctuates a little while the semi-conductor material is being heated up. After the seed crystal is melted through, the travel motion of the heater coil is initiated. The generator frequency jg is controlled to steadily increase up to the value indicated at location x corresponding to the normal diameter of the semiconductor rod. At first the heating current 1;,

is kept at the value obtaining at the starting location x Since subsequently, due to the increase in diameter of the semiconductor material being melted, a better couplinrg.

ofthe heating coil with the melting zone comes about, the heating current I may at first be somewhat reduced but must soon again resume its increase because, with an increase in diameter, greater quantities of material must be melted and the losses due to heat radiation increase. Thereafter the current gradually increases up to the value required at the location x In tests made in practice with this method, a semiconductor rod of 18mm. diameter and a seed crystal of 4.5 mm. were used. The distance between points x and x had a length of 45 mm.

The anode current of the high-frequency genera-tor, pro

portional to the heating current, was 0.45 amp at location x and 0.52 amp at location x The frequency fg at location x was 3,785 'kilocycles per second, and at the location x was 4,000 kc./s..

The apparatus shown in FIG. 6 is suitable for performing the above-described method. I The semiconductor rod 102 with a seed crystal 103 fused thereto, is mounted in two holders 106 and 107. An induction heater coil 104 surrounds an axially narrow portion of the semiconductor rod to produce a melting zone 107. Coil 104 is preferably formed as in FIG. 4. The heater coil 104 is energized at its terminals 108 from a high-frequency generator 100, whose terminals 110 are connected to a suitable current source. The frequency of the generator 109 is adjustable by means of a rotary knob 111. The generator 109 may correspond to the one described above with reference to FIG. 2.

A capacitor 112 is connected parallel to the heater coil 104 for compensating the reactive current and forms a resonant heating circuit together with the coil 104 as explained above. The heater coil is fastened on an insulating carrier 113,.which can be moved parallel to the longitudinal axis of the semiconductor rod with the aid of a rack 124 and a pinion 123. The pinion is driven from a reversible electric motor 116.-

In principle, there is no difference between moving the heater coil along a fixed semiconductor rod, and displacing the semiconductor rod while the heater coil remains stationary. Analogously, there is no difference, in principle, between displacing the upper or the lower rod holder when the two rod ends are to be moved closer to, or away from, each other tojelfect stretching or contracting of the molten zone. In the present case, in which a seed crystal of considerably smaller cross section is fused to the lower end of the semiconductor rod, it has been found preferable, however, to operate with a constant travelling speed of the heating coil while the lower rod holder is kept at rest and the upper rod holder 106 is displaced in the axial direction of the rod. This is advantageous because the effect of the thicker rod portion is more prokept in rotation during the zone melting operation in known manner.

The anode or plate current of the high-frequency generator-109 depends upon the power consumption of the heating circuit, and hence upon the diameter of the semiconductor rod, because a change in rod diameter effects a change in the inductive coupling between coil 104 and molten zone 105. Such a variation in plate current of the high-frequency generator is indicated by a measuring instrument 117 and causes a corresponding voltage drop in a resistor 118 which corresponds to the resistor 13 of FIGS. 1 and 2. The resistor 118 is so adjusted that the plate current occurring at the desired diameter of the semiconductor rod produces a voltage drop in resistor 113 equal to the reference voltage of a battery 119. Under such compensated condition, the polarized relay 120 is inactive and the motor 112 is at rest.

tWhen the rod diameter varies, the generator current and hence the voltage drop of resistor 118vary accordingly. The relay 120 is energized and closes its contacts 121 in one or the other direction depending upon the direction in which the rod diameter departs from the desired rod diameter tendsto become too small.

value. As a result, the motor 122 is energized to run in the proper direction. The motor drives the rack 124 to move the upper rod holder 106 upwardly or downwardly. By properly polarizing the relay 120 and the motor 122, the motor whenever energized will run in the direction required for stretching the molten zone when it tends to become too thick, and for widening the zone when the In this manner, an accurately defined and predetermined diameter of the semiconductor rod being processed is constrainedly obtained.

As explained, the just-mentioned regulatory effect must not occur in a certain range, namely in the seed crystal,

, and at the location where the seed crystal is fused to the semiconductor rod. For this purpose, the parameter values, namely the adjusted generator frequency and the voltage of the compensating battery 119, normally correlated to the normal rod diameter, must be changed to different values for operation in the just-mentioned range where maintenance of a constant cross section is not desirable. That is, care must be taken that the generator frequency and heating current assume the performance characteristic illustrated in FIG. 5 and explained'above. The generator frequency can be varied, for example, by parallel connected capacitances. The heating current can be variedby an auxiliary voltage added in the positive or negative sense to the voltage of the compensating battery 119. In the illustrated embodiment according to FIG. 6, a group of capacitors 125 are connected in parallel relation to the frequency-determining tank circuit of the highfrequency generator 109. The selectively operable capacitors 125 are connected to the terminals 126 of the generator 109, the same terminals 126 being also indicated in FIG. 2. A stepping switch, operating preferably in synchronism with the drive for displacing the heating coil,

connects the capacitors 125 to the terminals 126 in accordance with a predetermined program. For this purpose, in the apparatus shown in FIG. 6, the drive motor 116 for the coil-displacing mechanism is connected by an electrornagnetically conitnolled clutch 127 with a shaft to which a contact arm 128 is connected. During travel, the arm 128 sequentially connects the capacitors 125 to the generator terminals 126 in synchronism with the travelling motion of the heating coil.

The same shaft is connected with a second contact arm 128 which likewise passes over a bank of contacts and thereby places auxiliary voltages selectively across the terminals 130. The auxiliary voltages are tapped off a voltage divider 131, energized by auxiliary direct-current voltage U In the above described tests, made in actual practice, fifty-two individual switching steps are provided in apparatus according to FIG. 6 for realizing the processing program according to the diagram shown in FIG. 5.

Obviously, the apparatus for performing the invention can be modified in various respects. For example, in lieu of employing additional capacitances in incremental steps, a rotary capacitor with correspondingly shaped electrode members can be used to afford a continuous variation. To mention another example of modification, the relation between the generator circuit and heating circuit may also be varied by maintaining the generator frequency constant and varying the capacitance of the resonant heating circuit. In this case, the capacitor 7 described above with reference to FIG. 2 may be designed as a variable capacitor as shown at 7a in FIG. 7, and this capacitor may be adjusted to different capacitance values by means of a drive of the type exemplified in FIG. 6, to secure a change in capacitance according to the desired program and in synchronism with the travel of the heating coil.

The effect obtained by varying the relation between the generator oscillatory circuit and the heating circuit may also be obtained by varying the coupling between these two circuits, while maintaining the other parameters of the two oscillatory circuits constant. For this purpose, the

coupling between the primary winding and the secondary winding of the inductance coil 21 in FIG. 2 may be made variable as is illustrated in FIG. 8.

After the travelling heater coil has passed beyond the above-mentioned range of program control, the clutch 127 (FIG. 6) is opened, Whereafter the contact arms 128 and 129 remain at standstill, in the illustrated positions. The release of the clutch 127 may be efie'cted electrically, for example with the aid of a limit contact 138 which is actuated by a member 139 mounted on the insulating carrier 113 of the heater coil 104. As long as the heater coil is located in the above-mentioned range where regulation for constant rod cross section is undesirable, the member 139 closes the contact 138 and thereby energizes the actuating magnet 127a of the clutch 127 from a current source 140. The program control according to FIG. will thus be effective only as long as the heater coil 104 travels upwardly from the seed crystal toward the point (x where the semiconductor rod has its full cross section. The coil carrier 113 may be provided with further switching means for cooperating with limit contacts in order to reverse the travelling direction of the heater coil (I refer to the report by E. Beuhler, entitled Contribution to the Floating-Zone Refining of Silicon, the Review of Scientific Instruments, volume 28, No. 6, June 1957, pages 453 to 460), for switching the heating power from a high value during the above-described regulation for upward travel of the coil, to a low value for downward travel so that only a glowing zone instead of a melting zone is maintained during downward travel. limit contacts actuated by the travel of the coil carrier may serve to put the rotation of the lower rod holder 107 into and out of operation. The individual control or relay circuits required for such purposes may be conventional and for that reason are not illustrated since they are not essential to the invention proper and are Within the scope of ordinary control engineering practices.

I claim:

1. In a method for floating-zone melting of semi-conductor materials in which a molten zone located between two end portions of a semiconductor rod is passed longitudinally along the rod, the molten zone being produced by inductive heating with the aid of a high frequency induction coil which surrounds the rod and is energized from a high-frequency generator, the tuned circuit employing said current for controlling a device which moves the two rod end portions relatively to each other so that the current in the high-frequency coil is maintained at a desired value, and further maintaining the rod diameter at an adjusted constant value by regulating the frequency of the high-frequency generator to control the heating power of the current supplied to the coil, the method being further characterized in that before starting the floating zone melting a thin seed crystal is fused to one end of the semiconductor rod, the seed having a smaller cross section than the rod, to minimize heat transfer therebetween, each pass of the melting zone through the semiconductor rod commencing in the seed crystal, the transition of the melting zone from the thin seed to the thick rod being controlled in accordance with a predetermined program for the generator frequency and the current supplies to the coil.

2. In a process for floating zone melting of an upstanding semiconductor rod to produce a rod of a given diameter, in which process a molten zone located between end portions of the rod is passed longitudinally along the rod, the molten zone being of a size which is capable of being supported by adhesion to a solid rod portion adjacent thereto, the molten zone being produced with the aid of a high-frequency induction coil spaced from, surrounding, and inducing heating currents in, said molten zone, and in which the'coil forms a part of a tuned circuit, and in which the rod diameter-dependent heating current is employed to control the axial spacing between the two rod end portions, so as to displace the Similar two rod end portions relatively to each other until the current flowing in said coil assumes a given datum value; the improvement therein which comprises further main taining the rod diameter at an adjusted constant value by regulating the frequency of the high-frequency generator to control the heating power of the current supplied to said coil.

3. An apparatus for floating-zone melting of semiconductor material to produce a semiconductor rod, comprising two holder means for opposite end portions of the rod to support the rod vertically, means for relative displacement of the holder means with respect to each other in an axial direction lengthwise of the rod, a high-frequency induction heater coil adapted for encircling the rod out of contact therewith, and to produce the floating molten zone therein, the coil having an axially limited dimension so as to produce a molten zone of a size sus-.

tainable by adhesion to the adjacent solid portion of the rod, means for relative displacement of the heater coil and the rod to cause displacement of the molten zone axially of the rod, a heater circuit, said heater circuit comprising said heater coil and a condenser connected across the coil, a high-frequency current generator having output terminals connected in said heater circuit, the current in said heater circuit being inherently subject to variation as the diameter of the encircled portion of the rod varies, due to a change in inductive coupling between the heater coil and the encircled molten zone, circuit means connected to said generator for carrying a current dependent upon the power consumed by the heating, an adjustable resistor connected in said circuit means, a control circuit means connected across the resistor, a constant direct compensating voltage source connected in said control circuit means in series with the resistor, a polarized relay connected for operation by said voltage source, means operated by said relay to operate the means for relative displacement of the holder means with respect to each other, the resistor being adjustable so that the generator output current existent with a predetermined desired diameter of the semiconductor rod produces in said resistor a voltage drop equal to the c0mpensating voltage of said direct voltage source and so that as a result the polarized relay does not operate said means for relative displacement of the holders with respect to each other unless there is a departure from said diameter, said high-frequency generator having frequency control means, the latter being operable to determine the rod diameter, and means automatically responsive to the approach of the coil to one of said holder means for varying the frequency of said generator according to a given program.

4. An apparatus for floating-zone melting of semiconductor material to produce a semiconductor rod, comprising two holder means for opposite end portions of the rod to support the rod upstandingly, means for relative displacement of the holder means with respect to each other in an axial direction lengthwise of the rod, a high-frequency induction heater coil adapted for encircling the rod out of contact therewith, and to produce the floating molten zone therein, the coil having an axially limited dimension so as to produce a molten zone of a size sustainable by adhesion to the adjacent solid portion of the rod, means for relative displacement of the heater coil and the rod to cause displacement of the molten zone axially of the rod, a heater circuit, said heater circuit comprising said heater coil and a condenser connected across the coil, a high-frequency current generator having output terminals connected in said heater circuit, the current in said heater circuit being inherently subject to variation as the diameter of the encircled portion of the rod varies, due to a change in inductive coupling between the heater coil and the encircled molten zone, circuit means connected to said generator for carrying a cur rent dependent upon the power consumed by the heating tive coupling between the heater coil and the encircled molten zone, a control'means operably connectedto said circuit means to automatically operate the means for relaquency control means operable to determine the rod diameter, and means automatically responsive to the approach of the coil to one of said holder means for varying the frequency of said generator according to a given program.

5. An apparatus for floating-zone melting of semiconductor material to produce a semiconductor rod, comprising two holder means (106, 107) for opposite end portions of the rod to support the rod vertically, means (123, 124) for relative displacement of the holder means with respect to each other in an axial direction lengthwise of the rod, a high-frequency induction heater coil (104) adapated for encircling the rod out of contact therewith, and to produce the floating molten zone therein, the coil having an axially limited dimensiontso as to produce a molten zone of a size sustainable by adhesion to the adjacent solid portion of the rod, means (114, 115) for relative displacement of the heater coil and the rod to cause displacement of the molten zone axially of the rod, a heater circuit, said heater circuit comprising said heater coil and a condenser (112) connected across the coil, a high-frequency current generator (109) having means for varying the frequency of the current output thereof and having output terminals (108) connected in said heater circuit, the current in said heater circuit being inherently subject to variation as the diameter of -the encircled portion of the rod is varying, due to a change in inductive coupling between the heater coil and the encircled molten zone, circuit means connected to said generator for carrying a current dependent upon the power consumed by the heating and consequently dependent upon said change in inductive coupling between the heater coil and the encircled molten zone, the generator operating at a variable frequency which is on a flank of the resonance curve of current or voltage in the heater coil, plotted against frequency, wherein the rod diameter is further maintained at an adjusted constant value by the high-frequency generator for regulating the frequency of said high-frequency generator to control the heating power of the current supplied to said coil.

6. An apparatus for floating-zone melting of semiconductor material to produce a semiconductor rod, comprising two holder means (106, -107) for opposite end portions of the rod to support the rod vertically, means (123, 124) for relative displacement of the holder means with respect to each other in an axial direction lengthwise of the rod, a high-frequency inductionheater coil (104) adapted for encircling the rod out of contact therewith, and to produce the floating molten zone therein, the coil having an axially limited dimension so as to produce a molten zone of a size sustainable by adhesion to the adjacent solid portion of the rod, means (114, 115) for relative displacement of the heater coil and the rod to cause displacement of the molten zone axially of the rod, a heater circuit, said heater circuit comprising said heater coil and a condenser (112) connected across the in the heating circuit increases and when the generator frequency is decreased the heating current and the heating power decline, said rod having a monocrystalline seed crystal fused thereto, the seed crystal being held by one of said holder means, the seed crystal having a cross section susbtantially narrower than that of the rod at the fusion junction, to minimize heat transfer from the molten zone through said rod and said seed crystal to said holder means, auxiliary means to diminish thickening of the junction of the seed crystal with said rod, comprising a device operating in accordance with a predetermined control program governing the relation of the generator frequency to the current supplied to the heater coil, said device comprising a plurality of selectable second capacitances (125) connected in parallel to each other and each connectable in series with said variable capacitance of the frequency adjusting means, connecting means for selectively connecting the second capacitances in series with said variable capacitance means, control means for automatically controlling said connecting means to connect said second capacitances selectively with said variable capacitance means correlatively with the means for relative displacement of the heater coil and the rod, a plurality of third (131) capacitances connected in said first control circuit means in parallel to each other and each connectable in series with said adjustable resistor, and means for automatically and selectively connecting the third capacitances in series with said adjustable resistor correlatively with the means for relative discoil, a high-frequency current generator (109') having out-put terminals (108) connected in said heater circuit, the current in said heater circuit being inherently subject to variation as the diameter of the encircled portion of the rod varies, due to a change in inductive coupling between the heater coil and the encircled molten zone, circuit means connected to said generator for carrying a current dependent upon the power consumed by the heating and consequently dependent upon said change in inductive coupling between the heater coil and the encircled molten zone, the generator operating at a frequency which is on a flank of the resonance curve of current or voltage in the heater coil, plotted against frequency, and so that when the generator frequency is increased, the current placement of the holder means with respect to each other. 7. The method of processing semiconductor materials,

comprising the steps of fusing to the end of a semiconductor rod a seed having a smaller cross section than the rod, supporting the seeded rod at two end portions, energizing a tuned resonant circuit having an induction heating coil by means of a high frequency generator while maintaining the coil in a position surrounding said rod so as to melt a zone on said rod, passing the coil along said rod, further maintaining the rod diameter at an adjusted constant value by regulating the frequency of the high-frequency generator to control the heating power of the current supplied to said coil, varying the relative distance between the two end portions in response to the eifect of said tuned resonant circuit upon the high frequency generator, and varying the frequency of the generator according to a given program when the coil is in the vicinity of the seed so as to correspond to smaller diameters.

8. An apparatus for floating zone melting processing of elongated semiconductor materials having a seed crystal of smaller cross section than the material fused at 'one end comprising, two spaced holder means for jointly supporting the semiconductor material by its one end and by the seed crystal, a high frequency induction heater coil adapted to encircle said material and forming part of a resonant tuned circuit, said coil having an axially limited dimension so as to produce a molten zone in said material of a size sustainable by adhesion to the adjacent solid portions of the material, regulating means for maintaining the cross section of the material and its diameter at a respective adjusted value; said regulating means including means for relative displacement of the coil and material to cause displacement of the molten zone, an adjustable high frequency generator having output terminals connected to said tuned circuit, control means responding to the frequency of said generator for displacing said holder means relative to each other along the longitudinal direction of said material whereby the rod diameter is maintained at an adjusted value; and means for varying the frequency of the generator away from the resonant frequency according to a given program when the coil approaches one of the holder means supporting the seed crystal.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES v Buehle rz The Review of Scientific Instruments, vol. 28, #6, June 1957, pp. 453 to 460.

Mittelmann 219 1O-7,7 Radio Engineering, Terman, third edition, 1947, pp. Wilmette 219-1077 5 522 525 I Boyd 1- 219-1077 Zur Stabilitat Senkrechter Sehmelzzoven, by Heywang Mika et a1 21910.77 Z. Naturforseh, 11a, pp. 238-243 (1956). Rummel et 21910-43 NORMAN YUDKOFF, Primary Examiner.

Emei 10 MAX L. LEVY, Examiner.

Emels 23-273 G. HINES, A. ADAMCIK, L. H. BENDER,

Keller et a1. 23301. X Assistant Examinel-s.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,265,470 August 9, 1966 Wolfgang Keller It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, line 10, for "S 64,492" read S 66,492

Signed and sealed this 1st day of August 1967.

(SEAL) Attestz V EDWARD J BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. IN A METHOD FOR FLOATING-ZONE MELTING OF SEMI-CONDUCTOR MATERIALS IN WHICH A MOLTEN ZONE LOCATED BETWEEN TWO END PORTIONS OF A SEMICONDUCTOR ROD IS PASSED LONGITUDINALLY ALONG THE ROD, THE MOLTEN ZONE BEING PRO- 